The red ribbon in the foreground shows how little wind there is. When the ribbon is strung out horizontally the wind is 8mph or more. If you watch the video you'll see that never happens. Sorry about the poor quality, it was shot with a cheap webcam and I compressed it a little...since I'm on dialup it still took forever to upload.
It's not powering anything yet but if I counted right and my assumptions are close, that's 36rpm in a 4mph (average) breeze. That's a good sign I think.
If you're using 6" PVC, consider cutting 2 inches out of the pipe first. Here's a pic of what I mean...
Removing part of the blade seems like a bad idea but it's really not. 1 inch from each blade doesn't reduce the total area much but it does change the shape of the blade and it will give the vawt a higher top speed. Also, if you're going 10' tall, don't do it exactly like Ben's - you'll want bearings on either end at the least.
Read my "quick vawt" diary entry if you get a chance. The way that's put together makes it really solid and it spins like crazy. It should be able to be scaled up pretty easily too, it just depends on the pipe you use and whether or not the blades flex too much...but then you could add extra brackets to keep it secure in several places down the length...
..and now I'm rambling and pulling the post off-topic...ahh!
So I guess you wouldn't want to cut just 2 inches from the 6" pipe like I said to do above - do that to your 4" pipes. I'm not sure what it should be for 6" pipes yet.